The invention relates to a method of producing glass bodies having regions with different optical refractions, consisting of a basic body and a coating layer which is sintered onto the basic body, covers the basic body at least partially and is made of a glass which, as it is doped, has a different index of refraction than the glass of the basic body.
The invention relates more specifically to a method in accordance with which the sintered glass body consisting of the basic body and a coating layer is further used as a preform with core and cladding glass for the production of an optical waveguide. It is however possible to use the glass body manufactured in accordance with the method of the invention in a very general way for the production of optical systems in which glass bodies having regions with a different reflection behaviour are required, such as, for example, lens systems. In such a case it is not necessary for the coating layer to surround the basic body as a jacket, but it must only cover a main region of its surface area whilst forming a sharply defined interface between the two glasses of a different reflection behaviour (basic body and coating layer). The glass body may, for example, be a rectangular parallelepiped and may be subjected to a mechanical further treatment.
The European Patent Application EP-182 250 discloses that by concentric combining of a glass rod and a cladding glass rod a preform for the manufacture of an optical fibre (rod-in-tube method) can be produced. A disadvantage of this method is that first two preformed bodies of glass must be made and subsequent thereto the interfaces between the components must be cleaned at great cost, before these two components can be sintered together. This method has the further disadvantage that sintering together of the two components must be effected at a relatively elevated temperature, (&gt;2000.degree. C.); this may result in unwanted changes in the geometry of the system, i.e. core glass and cladding glass flow already, so that the boundary between these glasses is no longer sharply defined.
From the European Patent Application EP-228 082 it is known to obtain a refraction index gradient in the core and cladding region of an open-pored green body by controlling the fluor partial pressure during the sintering operation. A disadvantage of this method is that no sharply defined boundaries between the different refraction index regions can be obtained. This patent application refers also to other methods of producing different refraction indices by a gradient of the green density, the particle diameter or the sintering ratio (change in the viscosity by co-doping in the core region). This method has the same limitations that no sharp boundaries between the different refraction index regions can be obtained.
The European Patent Application EP-249 230 discloses a method in which from two differently doped rods which are manufactured by means of a VAD procedure (vapour axial deposition) and sintered into compact glass a first rod is formed into a tube by means of an ultrasound drill and then the second rod is stretched and subsequently inserted into the tube, whereafter this system is sintered together.
This method has the disadvantage that the mechanical treatment of a sintered glass is very costly and complicated and that error-free interfaces between the rod and the tube cannot be guaranteed with certainly.